The HomePlug Powerline Alliance was founded in 2000 to promote and standardize networking over power lines. This powerline networking trade association has developed successive specifications for both broadband applications such as HDTV and low-speed command and control applications such as home security. It has certified a large number of products from multiple vendors.
Due to problems with early powerline technology propagating signals effectively across electrical phases in a house, there is a perception that a powerline is an unreliable solution which only works in newer houses, or over short distances. This problem was effectively solved with HomePlug 1.0, and succeeding specifications such as HomePlug AV have made further improvements in whole home coverage.
Among other things, HomePlug allows the use of Ethernet in bus topology, which is very desirable in some circumstances. This is achieved by use of advanced orthogonal frequency division multiplex (OFDM) modulation that allows co-existence of several distinct data carriers in the same wire. The use of OFDM also allows turning off (masking) one or more of the sub-carriers which overlap previously allocated radio spectrum in a given geographic region. In North America, for instance, HomePlug AV only uses 917 of 1155 sub-carriers.
The HomePlug powerline alliance currently has defined the following specifications:
HomePlug 1.0—Released June 2001—Specification for connecting devices via power lines in the home. Theoretical speed of 14 Mbit/s.
HomePlug AV—Released December 2005—Designed for transmitting HDTV and VoIP around the home. Theoretical PHY data rate of up to 189 Mbit/s.[1]
HomePlug AV2—under development—Targets evolving home network requirements with usable throughput speeds up to 600 Mbps and more robust coverage.
HomePlug Access BPL (BPL)—under development—A working group to develop a specification for to-the-home connection.
HomePlug Command & Control (HPCC)—Released Oct. 9, 2007—It's a low-speed, very low-cost technology intended to complement the alliance's higher-speed powerline communications technologies. The specification enables advanced, whole-house control of lighting, appliances, climate control, security and other devices.
The HomePlug AV specification was designed to provide sufficient bandwidth for applications such as HDTV (high definition TV) and VoIP (voice over IP). Utilizing adaptive modulation on up to 1155 OFDM sub-carriers, turbo convolution codes for error correction, two-level MAC framing with ARQ, and other techniques, HomePlug AV can achieve near the theoretical maximum bandwidth across a given transmission path. Key distribution techniques and the use of 128 bit AES encryption are specified as mandatory in the specification. Furthermore, even the interception of encrypted data exchanged between HomePlug AV devices poses a “significant challenge” for an attacker due to the adaptive techniques used to modulate the signal between two given points.
In addition to the HomePlug family of standards, other power line communication standards exist, such as X10, G.hn, G.hnem (under development), etc. Various standards may not be interoperable. For example, HomePlug uses turbo coding forward error correction while G.hn is proposing low density parity check (LDPC). The two are incompatible and supporting both techniques would make HomePlug chips large and expensive. Differences in Forward error correction (FEC) are one of the reasons why HomePlug 1.0 devices do not interoperate with HomePlug AV devices. Though it is possible to build a backward-interoperable device, the complexity may be prohibitively expensive.
The Homeplug AV standard specifies PHY/MAC communication techniques over powerlines. Some key aspects of the specification are tabulated in Table 1.
TABLE 1Key Homeplug AV ParametersBAV ParameterValueNominal sampling frequency75 MHzFFT length3072Occupied carrier range74-1228, out of which 917 are used andthe rest are “masked”Length of preamble symbol3840 samples (51.2 us)Number of header symbols½